The primary new feature of 3.0.0 is the ability to run under multiple platforms. FileZilla 3 does not only run under Windows, but also under Linux, *BSD, Mac OS X and many more. In addition, the internals of FileZilla have been streamlined and modernized.
To accomplish this, FileZilla 3 has been completely rewritten from scratch during the past three and a half years.

Unfortunately not all features currently implemented in the old 2.x versions could make it into 3.0.0. However most of them will be implemented in a future version. In addition, FileZilla 2 and 3 can coexist on the same system in case you depend on a feature currently in 2.x

As usual, please report all bugs with the new version in the bug tracker.